SYMINGTON, W. A. - Exxon Production Research Co., and HIGGINS, J. W. - Elf Exploration Co.

Abstract: Hydrocarbon Systems Analysis in the Diana Basin, Gulf of Mexico

Hydrocarbon systems analysis helps estimate hydrocarbon trap charge and, in some cases, phase (gas/oil). The analysis depends on understanding the entire hydrocarbon system, including source rock distribution, the volumes and timing of hydrocarbons generated, the vertical and lateral migration pathways from the source, and the distribution of traps along those pathways. This presentation describes the application of this technology to the Diana Intraslope Basin in the western Gulf of Mexico.

The Diana Basin is located 160 miles south of Galveston, Texas, in over 4500 feet of water. The hydrocarbon system in the basin consists of Lower Tertiary to Tithonian source rocks actively charging Plio-Pleistocene reservoirs. The source rocks are poorly imaged seismically, and the estimation of source age is from geochemical interpretation of the reservoired hydrocarbons and seeps. Discoveries in the basin include the Diana oil and gas field, the Hoover oil field, and the South Diana gas field. Trap styles in the basin include stratigraphic/structural traps along the basin margin (Diana, South Diana) and drape-related anticlinal closures in the central portion of the basin (Rockefeller, Hoover).

Hydrocarbon systems analysis was undertaken following the largely disappointing results of drilling at the Rockefeller Prospect in 1995.The Rockefeller discovery, located in the center of the basin, found a full gas saturation in only one of several objective sands, and the gas was almost entirely biogenic, implying that migration pathways for thermogenic hydrocarbons into the prospect were inadequate.A fault, which bisects the Rockefeller prospect with over 700 feet of throw at depth, had been thought to be the migration conduit for thermogenic hydrocarbons from deeply buried source rocks.At the time Rockefeller was drilled, Diana was the only other drilled structure in the basin. Several additional prospects, including Hoover, were thought to rely on the Rockefeller fault, and similar faults, being migration pathways. It was therefore necessary to address the question of migration adequacy on a sub-basin scale.

Potential vertical migration conduits were interpreted from spec 2-D and Exxon/BP 3-D seismic data.The analysis identified likely vertical migration conduits along the salt-sediment interface at the periphery of the basin. Faults associated with the basin-rimming salt appear conducive to vertical migration due to their frequent reactivation as the salt ascended. Lateral migration was analyzed for the three major reservoir objectives in the basin. The stratigraphic relationship among these horizons is summarized in fig. 1. The analysis utilized proprietary migration software, which identifies the drainage cell associated with each trap on an objective horizon. Combining this with the interpretation of likely vertical migration conduits, the possibility of vertical migration within each drainage cell, and the fill and spill relationships among the traps on each objective horizon were determined.The objective horizons were also back-stripped and de-compacted for paleo-drainage analysis.

Example maps showing present-day drainage cells and spill pathways for the "A50" and "P1:10" reservoir horizons are shown in fig. 2-3. These indicate that the Rockefeller drainage cell has no access to salt-related faults, while the Hoover and South Diana drainage cells can access these faults. Even in the absence of a detailed model for the deep thermogenic source rocks, this configuration of migration pathways explains the discovery at Diana, and the disappointing results at Rockefeller. This configuration also permitted forecasting later successes at.Hoover and South Diana.The resulting understanding of migration pathways in the basin can be summarized as follows:

- Because of its isolated drainage cell, the Rockefeller prospect relies solely on the Rockefeller fault for thermogenic hydrocarbon migration. Given that the fault does not intersect deeper source rocks, this explains why Rockefeller contains only biogenic gas.

- Hoover, South Diana, and other prospects have viable migration pathways for thermogenic hydrocarbons which are independent of the Rockefeller fault.

- Hoover and South Diana are likely to be charged with thermogenic hydrocarbons from a common vertical ascension point along the South Diana fault.

This understanding of migration pathways in the Diana Basin is supported by the eventual discoveries at South Diana and Hoover. Hydrocarbon system analysis provided the critical technical reason to pursue Hoover, and will be a key element of Exxon's risk analysis for additional prospects in the basin.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England